ABSTRACT
Groundwater has been identified as the world’s largest
resource that can be easily accessed especially when confronted with water
shortages as a result of low river discharges. Successful execution of
groundwater exploration using geophysical methods requires knowledge on the
relationships that exist between hydrogeological and geophysical parameters.
This makes data interpretation and delineation of groundwater zones easier,
thereby avoiding the drilling of marginal holes. This study was carried out in
the Kwabre District and was aimed at establishing a relationship between
airlift yields and electrical resistivity parameters. Three methods, viz.
borehole log analysis, cumulative resistivity method and the drilling rate with
respect to formation analysis, were employed to obtain resistivity (rho) and
thickness of the saturated zones from borehole logging and vertical electrical
sounding data. The obtained resistivity and saturated thickness were used to
compute transverse resistance (Tr) and longitudinal conductance (Sc). Each of
these parameters (i.e. Tr and Sc) including the resistivities of the saturated
zones were further correlated with airlift yields of the boreholes to evaluate
their relationships. Also, the airlift yields were used to create a yield map
of the district. Correlation results obtained from the borehole log analysis
and cumulative resistivity method suggest there is no relationship between the
airlift yield and the resistivity parameters. On the other hand, results
obtained from the drilling rate with respect to formation analysis showed that
the yield versus resistivity, longitudinal conductance and transverse
resistance are related by 60.85%, 57.16% and 50.25% respectively. However,
validations of the associated models were very poor in predicting measured
airlift yield values. Thus, further studies may be required to improve and
validate the method. This study also provided useful information on the
variation of airlift yields in the district.
CHAPTER ONE
INTRODUCTION
1.1 Background
Groundwater is known to be the world’s largest accessible
storage of fresh water. It is considered as the logical resource to turn to
when faced with water shortage which may arise from low and variable river
discharges (Ifabiyi et al., 2016). The convenient nature and occurrence of
groundwater have rendered it less expensive for treatment when exploited and
could be developed at a desired location and at a reasonable cost.
The demand for adequate, good-quality water has increased
extensively due to awareness and technology. Therefore, many people rely on the
exploration and exploitation of groundwater, which is one of the valuable
natural resources for sustenance of life. Its quality is good for human
consumption; hence, it is the most preferred choice in various households and
communities.
In the Kwabre District of Ashanti Region, groundwater has
proven to be the most reliable source of water for household, agricultural and
some industrial purposes. It has over the years, conveniently, served the
inhabitants of the district during the dry seasons when surface water runs out.
Groundwater consumption in the district has saved the inhabitants from
contracting some water-borne diseases that could be obtained from the
consumption of surface water, which is usually exposed to the glare of
potential pollution. As the occurrence of groundwater depends on the nature and
type of aquifer, the shallow aquifer system in the district permits groundwater
to be easily tapped by hand-dug wells and boreholes.
Due to the growing needs of groundwater in most areas
including Kwabre District, many methods have been used for its exploration.
These methods include water witching (dowsing), fracture mapping, direct
drilling, borehole logging and the use of geophysical techniques. Today, one of
the most efficient ways of identifying groundwater is the use of geophysical
methods. These methods include but not limited to, electrical resistivity,
seismic, electromagnetic, gravity and magnetic. Out of all the methods, the
direct current resistivity method is a common tool used in groundwater survey. This method can be successively employed
for groundwater exploration where a good electrical resistivity contrast exists
between the saturated and unsaturated layers. The vertical electrical sounding
with dipole-dipole array as a low-cost technique and as an authentic tool in
groundwater exploration, is more suitable for hydrogeological surveys in both
sedimentary and hard rock terrains. It has been used to successfully map
vertical variations in resistivity with depth, determine the depth to the water
table, thickness of the saturated zone, delineate structures that serve as
hosts for groundwater accumulation, map overburden thickness, determine the
depth to bedrock and provide an understanding of the geometry of aquifers
(Ifabiyi et al., 2016). This technique employs collinear arrays of electrodes
designed to input a 1-D vertical apparent versus depth model at a specific
observation point. Using this technique, a series of potential differences are
acquired at successful greater electrode spacing while maintaining a fixed
central reference point. The potential difference measurements are
propositional to the changes in the deeper subsurface (Cardimona, 2002).
Vertical electrical sounding (VES) technique for groundwater exploration has
proven reliable in many areas. For instance, it was used in delineating
groundwater zones in an arid region in Iran (Nejad et al., 2011). It has proven
very popular with groundwater prospecting and engineering investigation due to
the simplicity of the technique and has been used to map groundwater bodies in
many places in Nigeria (Jatau et al., 2013).
Although VES has proven useful and shown to be one of the
most reliable techniques for groundwater siting, unsuccessful and marginal
holes are still encountered; hence there has been studies trying to improve on
the success rate. Like many others, this study is investigating the use of VES
data to estimate the yield prior to drilling by creating a model that relates
yield to VES data. This model is to show that, results obtained from VES survey
can further be used to estimate the potential yield of boreholes and,
therefore, enhance selection of VES points for drilling. In so doing, it would
help improve the success rate in drilling boreholes and cut down the losses
from drilling dry and marginal holes.
The main purpose of this research is to establish a possible
relationship between borehole yield and some electrical resistivity parameters.
The specific objectives are to:
* Develop a yield map of the area,
* Determine resistivity of the saturated zone,
* Determine transverse resistance and longitudinal conductance
of the saturated zone, and
* Establish correlation between the yield and electrical
resistivity parameters
1.3 Scope of Study
The research is limited to Kwabre District of Ashanti Region
and concentrated on creating models that relate yield to vertical electrical
sounding data. Secondary data consisting of dipole-dipole siting data, borehole
logs and borehole construction data were used. The ArcGIS software was used to
produce the yield distribution map of the area. Comparing the VES data with the
borehole logs, resistivity and thickness of the saturated zones were obtained
and used to compute transverse resistance and longitudinal conductance. Also,
airlift yields obtained from the borehole logs were correlated with each of the
electrical resistivity parameters to determine the relationship between them.
This thesis comprises five chapters. The background, problem
statement, justification and scope of work are presented in chapter 1. Chapter
2 includes a review of literature on previous studies done in relation to the
study while Chapter 3 provides a brief description of the study area and
explains the methods employed in the study. The study results and discussion
are presented in Chapter 4. Lastly, conclusion and recommendations based on
findings from the study are provided in Chapter 5.
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